Charge coupled devices (CCDs) are the detectors of choice for a large number of terrestrial astronomical applications in the visible and near infrared, where high sensitivity and low noise are the important considerations. In addition to terrestrial use, such devices are now being (or will soon be) flown on several space-borne satellites and missions. In 1976, CCDs were selected as the detectors for the Hubble Space Telescope (HST) Wide Field/Planetary Camera (WF/PC). CCDs also are the detectors selected for the camera on the Giotto Mission to Halley's Comet, the Solid State Imager (SSI) on the Galileo Orbiter, and for the solar imaging cameras for the Shuttle-based Solar Optical Telescope (SOT) mission. Both Galileo and Giotto are visible imaging missions, while the devices for the SOT mission require response into the ultraviolet (UV), and the devices for WF/PC require response shortward to Lyman-.alpha. (.lambda.=1216 .ANG.). For these latter devices, the UV response was enhanced by using an evaporated organic phosphor film coating (coronene) which converts UV photons to the visible.
In the past several years there has been a surge of interest in the areas of UV, XUV, and x-ray instruments, imaging, and astronomy. This interest, coupled with an unexpected short wavelength instability of the quantum efficiency of the thinned backside illuminated CCDs used by WF/PC, has stimulated a detailed investigation of the short wavelength response (1 .ANG.&lt;.lambda.&lt;5000 .ANG.) and performance potential of the CCD as a detector for this spectral region. It has now been demonstrated that uncoated devices are useful detectors with potential to achieve very high quantum efficiencies over the entire spectral range of 1 to 11,000 .ANG., (i.e., from the soft x-ray to the near IR).
The fundamental parameters which ultimately determine CCD performance are:
(1) Read Noise PA1 (2) Charge Transfer Efficiency (CTE) PA1 (3) Quantum Efficiency (QE) PA1 (4) Charge Collection Efficiency (CCE)
At the present stage of development, it is now possible to consistently fabricate CCDs which have low read noise (in the 4 to 15 e.sup.- range) and excellent CTE performance (&gt;0.99999). The means of achieving high QE and CCE, of which the CCD is capable, is the subject of this application.
As will be described more fully hereinafter, the present invention utilizes backside illumination of any thinned CCD, whether it be four-phase, three-phase, two-phase, uniphase or virtual-phase in a manner which yields theoretical quantum efficiency performance for blue, ultraviolet, far ultraviolet and low energy x-ray wavelengths. However, the specific embodiment to be described herein by way of example, and not by way of limitation, utilizes the three-phase CCD.